Copying DEC VAX set up disks rx50, help

[Allison]

>
>Subject: Re: Copying DEC VAX set up disks rx50, help
>   From: "Chuck Guzis" <cclist at sydex.com>
>   Date: Mon, 05 Mar 2007 16:19:46 -0800
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>
>At sometime today,  Allison wrote:
>
>> It's not 300 data rate it's 250!


Thats out of context.  When I say 250 data rate I refer to the nominal 
speed drive it was intended for (300 RPM) not the 1.2mb aberation.
I helps to know what the source media is for and in the case of DEC
the formats are limited in number used are easy to get data about 
and recognize. Of course the file organization on that media can be 
far more varied in the DEC cases A partial list includes (on floppy):

POS, RSX-11, DOS, RT-11, RSTS, Files-11, ultirx-11
VMS, VMS saveset, ultrix/VAX,
formats used for MIPS based systems,
OS8/78/278 
DECmate WPS systems.
VT180 Robin (CP/M on RX180)

I likely missed something.  However, the media if floppy based is 
only RX01/2 (8"), RX50, RX33, and late in the game RX25(3.5" 720k) and 
(RX26 3.5" 1.44m).  

I understand reading a DD 5.25" disk on a 360rpm fixed speed 1.2mb drive
will give 300 as data rate. I tend to treat and sequester all 1.2/1.6mb
5.25 disks away from all other rates as they are usually PC application
with an exception outside the PC application space (RX33 for example).
So the PC I have for reading disks is a older 486/66 with three drives
on two controllers.  The drives are standard 3.5" that does both 720 and
1.44m, 360k (48tpi FD55BV), FD55F for 96tpi DD (formats like RX50). I 
do not bother with PC 1.2MB format and the only place it appears here 
is DEC RX33 disks that my PDP11[Qbus 11/73 with an RQDX3/RX33) handles 
as native.  Even that PDP-11 is an unusual nonDEC configuration as it has 
RX02, RL02, RX50/33 and RD52(2 of them) in a rack mount format.  

I have Imagedisk, Fcopy, and teledisk and it runs dos/win95 for networking.
this makes for a predictable PC based system that is well behaved reading 
formats like RX50.  It was easier to put an old 486/66 minioard in a small 
tower with all the disks as I had it and the ISA floppy controllers, NIC and 
all needed.  It has a small 420mb disk for local storage and programs
as that all that is needed sice with networking I can mount the selected
drive and copy from there usless it's an nonPC format.

For those times when it's not PC based there is a NS* crate with NS* 
controller for hardsector two of my intelligent controllers for 
8"/5.25"/3.5" formats that occur in the CP/M and NS*dos worlds.


>Well, let's see what the numbers say.
>
>360 RPM is 6 revolutions per second or 0.167 second/revolution
>300 RPM is 5 revolutions/sec or 0.200 seconds/rev.
>
>Thus, the raw capacity of a 300 RPM drive at 250Kbps is 50,000 bits 
>or 6250 bytes.  The raw capacity of a 360 RPM drive at 250kpbs is 
>41667 bits or 5208 bytes.
>
>On the RX50, since there are 10 sectors of 5120 bytes, we have 
>
>6250-5120 = 1130 bytes for gaps and ID marks, CRCs, etc. on a 300 RPM 
>drive.  On a 360 RPM drive, one has 5208-5120 = 88 bytes for all that 
>overhead.  Not considering gaps yet:
>
>There are 10 bytes for a IDAM: A1 A1 A1 FE cc hh rr nn cr cr, where 
>cc is the cylinder number, hh is the head number, rr is the sector 
>number, nn is the sector length code and cr cr is the CRC for the 
>IDAM.  To that, add the data preamble of 4 bytes A1 A1 A1 FB and a 2 
>byte CRC and you have (without gaps) 15 bytes per sector or 150 bytes 
>for a 10-sector track--and we're out of space already at 360 RPM 
>before we can add the barest of gaps (e.g. 12 bytes of 00 preamble 
>for the IDAM, 12 bytes of 00 preamble before the DAM...
>
>You just can't do it at 250Kpbs and 360 RPM.  Heck, 10x512 is a 
>fairly tight fit at 300 RPM.
>
>Cheers,
>Chuck

I agree!  The trouble maker is the interrecord gaps when added to the data 
fields and other "position" fields like IDAM space becomes very tight.
DEC solved this partially by truncating the end of track gap and the 
Index gaps making it harder to read with a FDC that was designed 
to expect them.


Allison